The production of hydrogen from thermochemical cycles is a science that has been evolving over the past thirty years. Several sulfur based thermochemical cycles that incorporate sulfuric acid decomposition are now known in the art, such as a Sulfur-Iodine cycle, a Hybrid Sulfur cycle, and a Sulfur-Bromide cycle. Starting back in the mid-70's, a process using sulfuric acid was developed by Westinghouse (Pittsburgh, Pa.), hereinafter the Westinghouse Sulfur Process. This process used thermal energy from a nuclear High Temperature Gas Cooled Reactor (HTGR) such as the Pebble Bed Modular Reactor (PBMR) for the decomposition of sulfuric acid or sulfur trioxide to oxygen, water and sulfur dioxide at elevated temperatures. The sulfur dioxide released during the decomposition is absorbed in water at about room temperature and sent to an electrolyzer. The sulfur dioxide and water is then electrolyzed to hydrogen and sulfuric acid in liquid form or sulfur trioxide in liquid form.
A more detailed view of the prior art is shown in prior art FIG. 1. As shown in the first step under reference number 2, the process forms sulfur dioxide through decomposition of sulfuric acid at elevated temperatures. This is called an oxygen generation step. The thermal energy required for this step is generally heat at a temperature above 600° C., preferably in the range of about 700 to 1100° C. The thermal energy is provided by any generator able to produce heat at that temperature level.
The reaction for sulfuric acid decomposition and oxygen generation in prior art FIG. 1 is:H2SO4→H2O+SO3→H2O+SO2+0.5O2.
This step is often carried out in concert with a High Temperature Gas Cooled Reactor (HTGR) such as a PBMR to supply heat to the process. Various methods are employed to transfer the heat from the nuclear reactor loop to the decomposition reactor. One approach would be to use a bed of alumina or zirconia heat spheres with a catalytic surface that is heated with hot gas from an intermediate loop that is in turn heated by the reactor loop. The catalyst is employed to make the decomposition reaction proceed more quickly to the equilibrium value predicted for the temperature.
The sulfur dioxide is cooled in a vaporizer in second step, reference number 4. The vaporizer cools the sulfur dioxide in a heat exchanger, converting it from gas to liquid. Thereafter, in reference number 6, residual sulfur dioxide is absorbed in a counter current flow of water at a temperature above 40° F. to remove SO2 from the O2. This is referred to as the oxygen recovery step. The system generally operates under increased pressures of about 200 to 1100 psi. In other methods, the pressure of the system in step 3 is increased to between 1450 and 1700 psi, thereby allowing the sulfur dioxide to dissolve in water at higher temperatures or condense as a separate phase.
The sulfur dioxide in water is moved to a hydrogen production chamber where hydrogen is produced in a lower temperature step, reference number 8. The hydrogen production chamber is often an electrolyzer, wherein the energy for the reaction is an electrical current. In this circumstance, direct current electricity of between about 0.17 and 1.00 volt is added to the electrolyzer to react the sulfur dioxide and thereby forming aqueous sulfuric acid and hydrogen.
The reaction for the hydrogen producing step in prior art FIG. 1 is:SO2+2H2O →H2SO4+H2.
The electrolysis step is generally performed at temperatures of about 20 to 200° C. The current density is about 200 ma/sq.cm at about 60° C. By design, electrolysis processes do not present spark sources. The temperatures of the electrolysis step are not potential ignition sources for the produced hydrogen.
The aqueous sulfuric acid by-product of the hydrogen production step then re-enters the vaporizer in reference number 10. The vaporizer must vaporize the sulfuric acid, thereby converting it from liquid to gas, for the cycle to be complete. The vaporized gaseous sulfuric acid is thereafter fed back into the oxygen generation system of 2, repeating the cycle.
Another hydrogen production process that has been in existence for years is a Sulfur-Iodine process by General Atomics. The General Atomics process utilizes iodine and sulfur dioxide to produce sulfuric acid, which is then decomposed to oxygen, water and sulfur dioxide. The iodine process generally uses high temperature thermal energy from a nuclear reactor (˜1000° C.) for the decomposition of sulfuric acid. The process is continually repeated in the aim of producing intermediate HI by-products from the reaction. The process produces hydrogen from the intermediate HI products of the sulfuric acid decomposition by reacting them under elevated temperatures. This hydrogen producing step is typically done at about 400° C.
The above processes, however, include significant energy requirements in that the sulfuric compounds utilized in the process require shifts in their physical state, namely from liquid to gas. In fact, the vaporizer is utilized twice in order to convert the physical state of the sulfuric compounds, reference steps 4 and 10. This not only increases the energy level required to perform the process, but reduces the lifespan of the catalyst during the decomposition procedure.
Thus, there continually remains a need to reduce the costs and increase safety levels of hydrogen production processes, especially those wherein a nuclear power plant provides the thermal energy for the process.